Alfa Flag

Letter A of the International Code of Signals, a white and blue swallow-tailed flag (blue fly, white hoist), flown singly to mean “I have a diver down; keep well clear at slow speed.” It warns nearby traffic that divers are in the water and that the vessel cannot maneuver freely. The rigid version is required under US navigation rules near diving operations, and the flag is the maritime counterpart to the recreational red-and-white diver-down flag.